LibWeb: Stub out a basic IntersectionObserver interface

Note there are a couple of type differences between the spec and the IDL
file added in this commit. For example, we will need to support a type
of Variant to handle spec types such as "(double or sequence<double>)".
But for now, this allows web pages to construct an IntersectionObserver
with any valid type.

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <AK/RefCounted.h>
+#include <LibWeb/Bindings/Wrappable.h>
+
+namespace Web::IntersectionObserver {
+
+struct IntersectionObserverInit {
+    DOM::Node* root { nullptr };
+    String root_margin { "0px"sv };
+    JS::Value threshold { 0 };
+};
+
+// https://w3c.github.io/IntersectionObserver/#intersection-observer-interface
+class IntersectionObserver
+    : public RefCounted<IntersectionObserver>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::IntersectionObserverWrapper;
+
+    static NonnullRefPtr<IntersectionObserver> create_with_global_object(JS::GlobalObject&, JS::Value callback, IntersectionObserverInit const& options = {});
+
+    void observe(DOM::Element& target);
+    void unobserve(DOM::Element& target);
+    void disconnect();
+};
+
+}